The invention relates to a horizontal deflection circuit for a cathode ray tube (CRT) that provides raster distortion correction, for example, inside pincushion or inside barrel raster distortion correction.
Horizontal deflection circuits commonly employ resonant trace and retrace principles. During retrace, the deflection circuit forms a high frequency parallel resonant circuit for storing energy in the deflection circuit from the flyback transformer primary winding. The retrace provides a fast return of the deflection current.
During the trace interval, a deflection switch provides a low frequency trace resonant circuit having a trace resonant frequency and formed by the deflection winding and the S-shaping capacitor. As a result, energy circulates from the yoke to the S-capacitor and back to the yoke in one trace period. The resonant circuit that includes the deflection winding and the S-capacitor produces a sinewave shaped component of the deflection current at the trace resonant frequency.
The value of the S-capacitor determines the S-shaping of the deflection current. Thus, the S-capacitor is selected for improving horizontal linearity. The voltage across the S-capacitor is cosine shaped. The amplitude is highest at the center of trace, when the deflection current is zero.
The correction of inner raster distortion requires modulation of the S-shaping as a function of the distance along the vertical axis of the raster display, i.e. from top to the center and from the center to the bottom of the raster. This is achieved by modulating the peak-to-peak amplitude of the component of the voltage across the S-capacitor that varies at the horizontal rate. The modulated voltage appears also across the horizontal deflection winding and causes a modulation of the deflection current. CRTs with deflection angles larger than 110 degree require the trace resonant frequency to vary, in accordance with the vertical position of the electron beam of the CRT, for providing either inside pincushion or inside barrel distortion correction.
In carrying out an inventive feature, an adjustable raster correction is accomplished by coupling an inductor-capacitor (LC)-network during a portion of the trace interval to the S-capacitor via a controllable switching transistor. The LC-network forms a second trace resonant circuit. The current shape in the second trace resonant circuit is approximately sinusoidal with a period equals to the conduction time of the controllable switching transistor. By controlling the conduction time of the controllable switching transistor, the frequency and the magnitude of the current in the LC-network is selected. The resulting deflection current shape is the sum of the controlled, approximately sinusoidal current in the LC-network and the current in the S-capacitor.
A deflection apparatus embodying an aspect, includes a deflection winding coupled to a retrace capacitance to form a retrace resonant circuit during a retrace interval, and coupled to a trace capacitor to form a first trace resonant circuit that includes the deflection winding and the trace capacitor during a trace interval. A first switching semiconductor is responsive to a synchronizing first signal at a frequency related to a first deflection frequency and coupled to the deflection winding for generating a deflection current in the deflection winding synchronized to the synchronizing signal. A modulation inductance is coupled in a second trace resonant circuit during the trace interval. A source of a modulation second signal at a frequency related to a second deflection frequency is provided. A second switching semiconductor is responsive to the first and second signals, having a first switching state during a first portion of the trace interval and a second switching state during a second portion of the trace interval. The first and second portions vary in accordance with the second signal. The second switching semiconductor is coupled to the modulation inductance for producing in the modulation inductance and in the trace capacitor a modulation current that modulates a voltage developed in the trace capacitor in accordance with the second signal for providing raster distortion correction.